1. Field of the Invention
This invention relates to electrically conductive or semiconductive products. In another aspect, this invention relates to electrically conductive or semiconductive products comprising polyolefins. In yet another aspect, this invention relates to electrically conductive or semiconductive products comprising polyolefins having improved resistance to the phenomenon of water treeing.
2. Description of the Related Art
Typical power cables generally comprise one or more conductors in a core that is generally surrounded by several layers that include, a first polymeric semiconducting shield layer, a polymeric insulating layer, a second polymeric semiconducting shield layer, a metallic tape shield, and a polymeric jacket.
A wide variety of polymeric materials have been utilized as electrical insulating and semiconducting shield materials for power cables and in other numerous applications. In order to be utilized in services or products where long term performance is desired or required, such polymeric materials, in addition to having suitable dielectric properties, must also be enduring and must substantially retain their initial properties for effective and safe performance over many years of service. For example, polymeric insulations utilized in building wire, electrical motor or machinery power wires, or underground power transmitting cables, must be enduring not only for safety but also out of economic necessity and practicality. It is easy to see the danger of a non-enduring polymeric insulator on building electrical wire, or the impracticality of having to replace underground transmission cables frequently because of a non-enduring polymeric insulation.
One major type of failure that polymeric power cable insulation can undergo, is the phenomenon known as treeing. Treeing generally progresses through a dielectric section under electrical stress so that, if visible, its path looks something like a tree, hence the name "treeing." Treeing may occur and progress slowly by periodic partial discharge, it may occur slowly in the presence of moisture without any partial discharge, or it may occur rapidly as the result of an impulse voltage. Trees may form at the site of a high electrical stress such as contaminants or voids in the body of the insulation-semiconductive screen interface.
In solid organic dielectrics, treeing is the most likely mechanism of electrical failures which do not occur catastrophically, but rather appear to be the result of a more lengthy process. In the prior art, extending the service life of polymeric insulation has been achieved by modifying the polymeric materials so that either trees are initiated at higher voltages than usual or the growth rate of trees is reduced once initiated.
The phenomenon of treeing itself can be further characterized as two distinct phenomena known as electrical treeing and water treeing.
Electrical treeing results from internal electrical discharges which decompose the dielectric. Although high voltage impulses can produce electrical trees, and the presence of internal voids and contaminants is undesirable, the damage which results from application of moderate A/C voltages to electrode/insulation interfaces which contain imperfections is more commercially significant. In this case, very high, localized stress gradients can exist and with sufficient time lead to initiation and growth of trees which may be followed by breakdown. An example of this is a high voltage power cable or connector with a rough interface between the conductor or conductor shield and the primary insulator. The failure mechanism involves actual breakdown of the modular structure of the dielectric material perhaps by electron bombardment. Much of the prior art is concerned with the inhibition of electrical trees.
In contrast to electrical treeing which results from internal electrical discharges which decompose the dielectric, water treeing is the deterioration of a solid dielectric material which is simultaneously exposed to moisture and an electric field. It is a significant factor in determining the useful life of buried power cables. Water trees initiate from sites of high electrical stress such as rough interfaces, protruding conductive points, voids, or imbedded contaminants but at a lower field than that required for electrical trees. In contrast to electrical trees, water trees are characterized by: (a) the presence of water is essential for their growth; (b) no partial discharge is normally detected during their growth; (c) they can grow for years before reaching a size where they may contribute to a breakdown; (d) although slow growing they are initiated and grow in much lower electrical fields than those required for the development of electrical trees.
Electrical insulation applications are generally divided into low voltage insulation which are those less than 1K volts, medium voltage insulation which ranges from 1K volts to 35K volts, and high voltage insulation, which is for applications above 35K volts.
In low to medium voltage applications, electrical treeing is generally not a pervasive problem and is far less common than water treeing, which frequently is a problem.
For medium voltage applications, the most common polymeric insulators are made from either polyethylene homopolymers or ethylene-propylene elastomers, otherwise known as ethylene-propylene-rubber (EPR).
Polyethylene is generally used without a filler as an electrical insulation material. Polyethylene has very good dielectric properties, especially dielectric constant and power factor. The dielectric constant of polyethylene is in the range of about 2.2 to 2.3 which is an acceptable value. The power factor, which is a function of electrical energy dissipated and lost, and therefore should be as low as possible, is around 0.0002, which is not only acceptable, but a very desirable value. The mechanical properties of polyethylene are also very adequate for utilization as medium voltage insulation.
However, polyethylenes are very prone to water treeing especially toward the upper end of the medium voltage range.
There have been attempts in the prior art to make polyethylene based polymers that would have long term electrical stability. For example, when dicumyl peroxide is used as a crosslinking agent for polyethylene, the peroxide residue functions as a tree inhibitor for some time after curing. However, these residues are eventually lost at most temperatures of electrical power cable service. U.S. Pat. No. 4,144,202 issued Mar. 13, 1979 to Ashcraft et al. discloses the incorporation into polyethylenes of at least one epoxy containing organo silane as a treeing inhibitor. However, a need still exists for a polymeric insulator having improved treeing resistance over such silane containing polyethylenes.
Unlike polyethylene which can be utilized, the other common medium voltage insulator, EPR must be filled with a high level of filler in order to resist treeing. When utilized as a medium voltage insulator, EPR will generally contain about 20 to about 50 weight percent filler, most likely, calcined clay, and it is preferably crosslinked with peroxides. The presence of the filler gives EPR a high resistance against propagation of trees. EPR also has comparable mechanical properties to polyethylene.
While the fillers utilized in EPR may help prevent treeing, they unfortunately will generally have poor dielectric properties, i.e. poor dielectric constant and poor power factor. The dielectric constant of filled EPR is in the range of about 2.3 to about 2.8. The power factor of filled EPR is on the order of about 0.002 to about 0.005, which is about an order of magnitude worse than polyethylene.
Thus, while polyethylene has good electric properties, and good mechanical properties, it needs improvement in water tree resistance. While filled EPR has good treeing resistance, it needs improvement in dielectric properties.
Therefore, a need exists in the insulation art for a polymeric insulation having good mechanical properties, good dielectric properties and good water treeing resistance.